Cool Things You Can Do With a Blue Laser: Reflection vs. Fluorescence

Alternate title: Interactions Between Light and Matter.

Warning: Light and matter are both seriously complicated stuff. I am going to try and simplify this whole thing a bit so that everyone can see some cool stuff. Yes, that means that some of the things below won’t be completely true.

Everyone has a red laser pointer, right? I remember when the price of these first started to go down. Maybe this was one of the first things I ordered through the internet. I still have this old beast of a red laser pointer (I like it because it uses AAA batteries rather than those button cell ones). Even though these red lasers are everywhere, you can still show some cool stuff with them.

The nice thing about lasers is that they produce light of just one color. So, what happens when just red light hits different surfaces? Just red light is reflected. Even if you shine a red laser at a blue piece of paper, only red light is reflected. Try it. The best way to see this is in a dark room. Take your red laser and just start pointing at stuff (but not people). The dot you see will probably always be red. If you do this in a bright room, you might trick yourself. Sometimes if you see a red dot next to some other color, your brain can trick you into thinking it is not red. Don’t be tricked.

How do you know the red laser is just red light? Get a pair of these glasses:

These are holographic diffraction grating glasses (they are pretty cheap too). I won’t give a super-detailed explanation of how they work. Instead let me just say that different colors of light “bend” different amounts when they pass through the lens — just like a prism, but much easier to use. If you look at white light through these glasses, you will see a rainbow of colors.

You can do two things. Put on the glasses and look at the red dot the laser makes on the wall. Alternatively, you can shine the laser through the glasses at the wall (this way everyone can see the effect). Do not shine the laser through the glasses into your eye. That would be foolish. Either way you do it, it should look something like this:

So the red laser only makes one color of light (red) and when you shine in on stuff, it is only reflects red. Why? Here is the tough part — like I said the interaction between light and matter is not so simple. However, suppose I were to model the interaction by saying that it was like the electrons were held to their atoms by springs. When light shines on the matter, it makes the electrons oscillate with the same frequency as the incident light. These oscillating electrons then re-radiate the same frequency of light. The combination of all these radiating electrons is what makes the effect that you see.

Here is a diagram showing the reflection of green light from some material. Notice that the electrons are the red balls connected to some other stuff (remember that electrons are always red). I am pretty sure this model came from something Richard Feynman said about light. It is probably in his book: QED: The Strange Theory of Light and Matter.

Green light comes in, green light leaves. What if I shine white light on some material that is red? Why does that look red? Perhaps the best thing to say is that the “red” material is much better at re-radiating the red incoming light than the other colors.

Next step. Get a green laser pointer. Yes, they are cheap too. Repeat the above experiment and what do you find? First, the light from the green laser is also just one color.

Keep the green laser out. Put on the spectral glasses. Look as you shine the laser around the room. Keep going. Try a whole bunch of different things. BOOM. Did you see that? Here is what I saw:

If you want to try this, use something plastic that is either orange or pink with a green laser. So, what is going on here? This isn’t just reflection, this is something else. How do I know? If it were just reflection, the only color would be green (same as the incident light). This is an example of fluorescence. Basically, in fluorescence, the light doesn’t just oscillate the electrons. The light excites the electrons to a higher energy level. Let me try to show this with a diagram.

Some things to note. Some of the electrons are excited to higher energy levels. When they go back down to the ground state, they produce light of a particular frequency (color) that is related to that change in energy level. All of the electrons don’t have the same changes in energy levels. Why? Probably because it is in a solid with bands of energy levels. The same thing happens in blackbody radiation.

So why doesn’t the red laser do this? Please don’t say that longer wavelength light doesn’t have as much energy. That isn’t quite true. Example: Which light has more energy per second, the long wavelength “light” from your local radio station (KSLU is 3,000 watts) or your 5 mW laser pointer?

Although the red laser doesn’t necessarily have more or less energy, it does have a different frequency than the green laser light. It turns out that an electron is more likely to change energy levels if it is perturbed with a particular frequency of light (or any type of perturbation really). This frequency is:

Here ν is the frequency of the perturbation and h is a constant (the Planck constant). So, green has a high enough frequency to make this happen for some materials — red not so much.

What about a blue laser pointer? These are also now cheap. You can get one for around $10. Here is what happens when I shine the blue laser around on stuff:

The green light just fluoresced some stuff, the blue light does it to just about everything. Why? Higher frequency means a greater energy level change. This means more things have a chance of making that fluorescence jump. What if you have something with an even smaller wavelength? What if it is an ultraviolet light? You can get one of these nice ultraviolet lamps, you can see all sorts of stuff that fluoresces.

But why don’t you see these fluorescing materials with plain old white light? White light has the lower wavelengths like blue in them, right? Yes, that is true. So yes, white light should cause fluorescence. However, you don’t notice it because those colors are already there from the source, too.

Some other cool materials

Really, this whole thing started with the blue laser. As I was sitting around the house, I couldn’t stop shining the blue laser on different things. Here is one of those things:

Green laser in white wine

Yes. Blue laser in white wine is not blue. After posting this picture on twitter, Jim Deane suggested that I try both red wine and olive oil. Yes. Those are both cool also. Here are some pictures.

Blue laser in olive oil

Blue laser in red wine

Pretty cool. Oh, you have to add some water to the red wine or the effect is very difficult to see. Also, you can use the green laser with the olive oil. Here is what that looks like:

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